I would like to thank Kirk Bryan at Princeton University for discussions that have led to the idea of developing a low-order model, Atsushi Kubokawa at Hokkaido University for valuable discussions and comments, and the anonymous referees for comments that helped improve the manuscript. Several figures are produced using the GrADS developed by Brian Doty at University of Maryland. This work was supported by grants from the Japanese Ministry of Education, Culture and Science, and from the Center for the Climate System Research of the University of Tokyo.
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